Scanning radio receiver



May 19, 1942. s. Y. WHITE SCANNING RADIO RECEIVER Filed Feb. 29, 1940 2 Sheets-Sheet l ATTORNEYS May 19, 1942. s. Y. WHITE SGANNING RADIO RECEIVER 2 ,Sheets-Sheet 2 Filed Feb. 29, 1940 ATTORNEYS Patented May 19 1942 UNITED STATES PATENT j 4OFFICE I SCANNIN RECEIVER v A sxaney Y. white, wumette, n1. i Application February 29, 1940, Serial N0. 321,378

(Cl. Z50-20) 14.Claims.

sion desired and adapted to explore a portion of the spectrum to locate a, carrier wave, and Eo stop exploringwhen a carrier wave isencou 'tered. This type of receiver is hereinafter referred to as a "scanning" receiver. The invention is in the nature of an improvement on the scanning receiver' disclosed and claimed in Serial No. 321,377, ledFebruary 29, 1940, in my name.- I n said applicationI a radio receiver is disclosed which is especially adapted for the reception of carrier Waves having a frequency higher than 20 megacycles and wherein inertialess scanning means are provided for vrapidly scanning a portion of the spectrum in which a carrier is expected to occur. Should there be several transmissions operative in that portion of the spectrum the receiver therein disclosed will lock on the first carrier encountered, which may not be the one desired.

Scanning receivers of the ordinary type have been suggested which stop on each carrier consecutively and remain in this condition until a complete series of dots which identify a particular carrier have been received. If this ,carrier is unwanted, the receiver resumesl the scanning conrier. the receiver remains receptive to the carrier until either the receiver or the carrier is turned oli'.

it is easily realized that this is a time-consuming process, and, when a large number of channels must be explored, a transmitter must be transmitting its identifying code for a very considerable time to insurev that the receiver has had time to locate it. It is one of the main objects of this invention to avoid this delay by identifying the signal with a tone and providing means whereby the receiver locks on a carrier having a predetermined tone This tone can be of sub-audio, audio or super-audio frequency. The sub-audio range is rather unattractive due to the long time interval required for one cycle and for lter voltage buildup, as well as the size and weight of the filter. The tone may well be in the audio range, but if it is desired that the tone be on all the time, some complication ensues from the necessity of putting a hole in the audio response curve so that the tone would not be heard suiliciently loudly to interfere with the speech transmission.

Receivers of the communication type often have a cut-oil of 3000 cycles as the upper limit of the audio band transmitted. The identifying tone frequencies may, therefore, commence at 4000 cycles, for example, and continue on up indeiinitely; say to 20,000 cycles or even higher. A further use of a receiver of this type would be to locate and properly tune in television signals which would be identified by the tone provided by the scanning frequency, which at present is of the order of 40,000 cycles, or by any synchronizing frequency present on the carrier. 'I'he term tone as hereinafter used in the specificationl and 4claims is intended to include all of the above mentioned frequencies.

Obviously this tone can be left on all the time with say 30% modulation, or could only be impressed on the carrier for the purpose of contacting the receiver, which should occur in one or two seconds. In this case the tone would modulate the carrier Arrangements could also be made so that the tone plus the voice at all times would add up to 100% modulation; that is, at a given period, if the voice energy were zero the tone would be 100% modulation on the carrier, but at anothertime if the voice energy were 100% the tone energy would be zero. This would be yfeasible due tothe extremely short time in tervals during which spe peaks reach 100% modulation, and the receiver can easily be made not to relinquish contact during these extremely small intervals. 7

The advantage of having the tone on all the time is especially marked in mobile work, where due to interference patterns on the ground or the attitude of a plane in the air, there may be short intervals where zero signal exists in the receiver antenna. After this condition ceases to exist, however, rapid and positive contact with the carrier must once more be obtained, and naturally, if the tone were not there, the receiver would not lock on the carrier.

The preferred way to utilize this tone is to provide a receiver of the type shown in the aforementioned application but in which the director device whichserves to eiect the locking on the carrier is normally inoperative and is only rendered operative to perform the locking functionl by a switching device which is responsive to a predetermined tone impressed on the desired carrier.

In acase where the tone is only impressed on the carrier for several seconds and then removed, it is obvious the proper carrier has been located and the tone preferably triggers off a lswitch which brings about the lock-on-carrier condition which will then be self-maintaining as long as the carrier wave is received.

It is a feature of the present invention inaddition to that described in the aforementioned application, that the sweep voltage is left on continually and is overcome by a much higher voltage generated by a director device which is energized by the carrier.

Each receiver of a group could be arranged to respond to either of two tone frequencies; one of these (for example 5,000 cycles) could be common to all the receivers, and the other (for example 7,000 cycles) could be specific to only one receiver. All receivers in the group would then lock on a carrier modulated with 5,000 cycles and such a carrier would then serve as a general call to all the receivers, while a carrier modulated with '7,000 cycles would act as a specific call to the one receiver only.

In the case of a complete communication system where private channels may be set up between two or more of the units in the group, i and it is desired to break up these conversations to give precedence to a general call, it would be necessary to have two receivers at each station, one of which would respond to the general call only, and the other respond to the individual call only. In that case, if one receiver were lworking on an individual call, a general call would actuate the other receiver at each station, and the latter receiver could be arranged to take over the receiving transducer from the individual channel, thus breaking up the private conversation and forcing everyone to listen to the general call.

It will be realized that once a receiver is designed to respond only to an identifying tone, it must not respond to a carrier it passes over, regardless of its strength, which does not have the exact tone identification for which the receiver has been designed. A feature to be guarded against in the construction of a receiver which rapidly scans a large number of channels is the building up of paralyzing transients which will in effect cast a shadow behind such a strong undesired carrier, so that the receiver will keep on sweeping, but will -be paralyzed until the excess voltage charges leak off, as for example from the grid circuits. This means that the desired station may be hidden in such a shadow so that the receiver will lack the necessary sensitivity to pick it up promptly when sweeping in that direction, but when returning in the sweep from the opposite direction will, of course, pick it up satisfactorily.

In high speed scanning receivers of the tone identified type the action of the automatic volume control device should be either extremely fast or extremely slow. If it has some intermediate value, it will cause the receiver to lose sensitivity for a time just after pasing over an undesired carrier. Since the receiver continues scanning regardless of the AVC voltage developed, there would be a certain time of AVC voltage decay during which the receiver will be quite insensitive, and unable to pick up a weak desired carrier which might be encountered. In scanning at the rate of several hundred channels per second, a high speed AVC action is rather diilcult to secure, and an extremely slow one is preferred which will build up only a negligible voltage in passing through an undesired carrier at these high scanning speeds by choosing large values for C9 and R18.

Requirements of the tone operated switching device, hereinafter referred to as the "tone switch, are severe. We may sweep at the rate of to 200 channels per second, equivalent vto passing over the entire broadcast band in cori--l condensers, and energize the director, which in.

turn must build up and charge its filter condensers, and the signal must be locked on and held, which requires that no transient surges can be allowed to build up, no matter how small, which on discharge would tend to cause release of the carrier once it had been identified and locked on.

'Ihe tone can be recognized and utilized by mechanical resonant systems, as for example, by a tuning fork; or by a resonant electrical circuit or several such circuits in cascade. The selectivity must be of such a high order that, if the circuit is tuned, for example, to accept 7000 cycles, no response whatsoever can be tolerated at 6000 cycles or 8000 cycles. It will be realized that the desired signal modulated at 7000 cycles may generate a voltage of one microvolt but an undesired signal modulated at 6000 cycles may generate a voltage of 100,000 microvolts and the receiver must not lock or even hesitate when passing through the 100,000 microvolt undesired signal. lI'he customary manner of securing this selectivity is to use as highly resonant circuits as practical and cascade them until the desired degree of selectivity is attained. The size and Weight of such a filter is considerable, and it has the further serious objection that the time interval required for full resonant voltage to build up is too long to permit the receiver to scan at a sufficiently rapid rate. It is one of the main objects of the present invention to provide a selective filter device which is light and compact and which responds sufficiently rapidly as to cause the receiver to lock on the desired carrier. In cases where it is desired that the receiver respond to a master tone that is common to a group of receivers as well as to an individual tone specific to one receiver only, it is a further object of the invention to provide two such filter devices whose rectified outputs are connected in series or in parallel.

Another object of the present invention is to provide a director device for locking the receiver on a desired carrier, and in whichl no voltage is built up while passing over an undesired carrier. For this purpose the director device is preferably provided with a driver tube which has a screen grid. The screen grid is normally held at substantially 20 volts to absolutely block the plate current until voltage is applied from the tuned filter switch circuit which raises the screen voltage to substantially +100 volts for normal operation of the director.

In the preferred embodiment of the invention, the director device varies the frequency of a local oscillator through the action of a control tube, and it is a further feature of the invention that the director is connected to the control grid of the control tube by a circuit having a large time constant. This circuit serves to filter out noise current components which appear in the director circuit. In the case of weak signals of the order of the noise level, a considerable noise voltage is developed in each branch of the director device. These branches are carefully matched and balanced and the noise voltages developed tend-to cancel out when the signal is properly tuned in: When a considerable amount of director voltage is required to maintain the receiver tuned to the signal. one branch of the director develops more voltage than the other so that the balance is destroyed and a very considerable fluctuatingnoise voltage appears superimposed on the direct current director voltage so extremely effective noise filtering is required.

When a desired signal is intercepted', it is properly tuned in and the function of the director is then to hold it in. In the next few milliseconds the sweep oscillator continues to generate a voltescasas 3- hasacondenlerlleonnectedacrossit. Bya properchoiceofthevaiueoftbiscondensen'a sharp cut oif. above 9000 cycles is obtained so,

that any tone frequency of 4000 cycles or higher ceas'estobeannoyingtotheusenevenwhenthe tone frequency is left continually on'during the speech transmission. It is found that a filter ofthistypewillallowtheusualrangeoftelephonic speechfrequencies to pass and a rapid attenuation above those limits.

The scanning device I4 comprises the vacuum tubesv'rlandv'rc, the piateoracn wbebeing age of a few millivolts which tends'to keep the receiver in a scanning condition. It is then only necessary' for the' director to generate a few millivolts of opposite polarity to keep the receiver locked on the carrier. As the frequency oi the sweep voltage is at a rather slow rate, a very mize the noise fluctuation voltages.

For a better understanding of the invention, reference is made to the following description connected to the grid of the opposite tube through tle condenser CI and C2 having a capacity of- 0.5 mfd., and the grids of the ter serving to block oif the direct current voltage.

. large time constant illter may be used to minitaken in connection with the accompanying drawings `in which:

Fig. 1 is a schematic circuit diagram in block form of a radio receiver embodying the invention; and

Fig. 2 is a schematic circuit diagram of the radio receiver of- Fig. l in which the circuits of several of the component parts are shown diagrammatically in more detail.

Referring to Fig. l the, receiving system is shown as comprising an antenna I connected to the tunable input circuit 2of a radio frequency amplifier 3, the circuit 2 being permeability tuned over a band of frequencies as indicated at 4. The output circuit of amplifier 3 feeds in turn a ilrst detector or mixing device 5, an intermediate 'frequency amplifier 6, a second detector 1, audio frequency amplifier 8, band pass nlter 9 and a translating device, such as the .earphones Il. If desired, the output level of the receiver may be maintained at a substantially constant level by an automatic volume control device II, which may be of any known type; The amplifier 3, mixing device 5, intermediate frequency ampliiler 6, detector l and audio amplifier 8 may be of any known type, as used in superheterodyne receivers for the reception of modulated carrier waves. The oscillator frequency is supplied to the mixing device 5 by an oscillator I2, whose frequency is controlled by a control tube I3 which is subject to the control of a sweep oscillator lI4 preferably generating a low frequency, such as one half cycle per second. The control tube Il is also subject to the control of a. director device I5 which is in turn controlled by a switch device I6 operated by one of the tone filters Il, Il' which areconnected in parallel to the audio frequency ampliner l.

.Referring howto Fig. 2, the audio amplier 9 is shown as comprising the tubes VTI and VTI which are resistance coupled, as shown, the latter tube also being provided with an extra plate i9 which cooperates with the tube cathode I9 to provide a diode rectifier. The output circuit of tube VT9 is connected to the earphones I0 by means of a band pass lter 9. The filter c omprises a push pull output transformer 20 which is designed to have a considerable amount of Resistor R'I and condenser C4 act as a filter to take voltage of square wave form at the plate of tube VTS and change it to a substantially sinusoidal wave form. At the same time the resistor R1, being 10 megohms, reduces the voltage from several hundred volts to i3 volts. A further advantage in making R1 so large is that it does not destroy the symmetry of the sweep oscillator I4. It will be realized that the half cycle oscillator covers the entire'scanning spectrum in one direction completely in one second. A control voltage is also supplied to lthe control grid of VT4 through resistor R9 and the director line 22 by the director device I5.

The director I5 consists of a director driver tube VTI whose control grid .is connected through lead 23 and condenser 24 to the output of the intermediate frequency amplifier 9, the cathode of the tube being connected to ground through the resistors RI4 and Ril lconnected in series. The tube cathode is connected to the B supply voltage source through the resistorv RI! and the B supply -is also connected through the resistor RIG to the screen grid 25 of the tube VTI and also to the plate of the switching tube VTII, as shown. The plate circuit of tube VTI comprises the low impedance primary windings 29 and 2l which` are coupled, respectively, to the secondary transformer windings of the circuits TCI, TG2, one of which is tuned to a frequency of from 5 to 10 kilocycles above the intermediate frequency, and the other of vwhich is tuned by the same amount below the intermediate frequency. These circuits feed the diodes VT2 to develop a differential voltage across the resistors RI and R2. As shown, one end of the resistor RI is grounded and one end of resistor R2 is connected by director line 22 to the control grid of tube VT4.

The oscillator is shown as comprising the tube VTI whose cathode is grounded and whose plate circuit is provided with a tuned tank circuit comprising thecoil LI and condenser 2l. It is found that by making the condenser 29 of ilxed value and varying the inductance of coil LI so as to change its permeability, as by means of a powdered ferro-magnetic core, as indicated at 29, substantially equal percentage tuning effects are secured throughout the band of frequencies.

It is found that the ordinary hydrogen reduced iron cores are not at all suitable for permeability tuning at frequencies of the order of leakage reactance and whose primary winding 100 mesacycies. However. has been found that ferromagnetic cores known under the trade name "Aladdinite are very satisfactory. These cores are made from a synthetically produced ferromagnetic mass powder consisting substantially wholly of magnetic oxide of iron in the form of minute particles, substantially all of which, as they appear under a microscope, are of generally rounded form. These particles are preferably molded into cores by mixing with several per cent of Bakelite" as a binder.

Considering the circuit which is grounded at the lower end of coil LI, the oscillator grid receives charging current through L2 representing the mutual inductance of the tickler coil with LI. The grid current at the end of L2 is thus exactly 180 out of phase with the circulatingcurrent in LI. Before reaching the grid of tube VT3, the current must pass through some series leakage inductance, represented by L3, which produces a reactance of rather small amount, but which is quite effective due to the very heavy current flowing through it. At the grid of VT3, energy is supplied which is so phased in relation to that of the plate tank circuit as to cause the tube to oscillate, but is not quite 180 phase angle from the plate. The amount of phasing may be varied as desired, by making L3 of the proper value, even, if necessary, by winding an additional uncoupled coil in series with the grid,vor by more loosely coupling L2, and by making this coil larger we can increase the leakage inductance to any desired amount without having any physical coil, as represented by L3. With this energy, preliminarily phased, as described, the grid energy of tube VT4 may now be further phased in the same direction by the series resistance R3.

For the purpose of controlling the frequency of the oscillations generated by oscillator VT3, the plate of tube VT3 is connected to that of a control tube VT4 and both plates connected to the mixing device 5 through a condenser 30. Control tube VT4 is provided with the usual supply voltage, its inner grid being given a normal bias of approximately -6 volts by connecting its cathode to the bleeder resistor 3| at a point approximately this number of volts above ground. This permits the operation of the tube at the mid-point of its control range, as determined by plotting a curve of grid bias against oscillator .frequency which results in an S-shaped curve,

so that we secure operation about the mid-point thereof. The voltage generated by the sweep oscillator I4 is arranged to be a total peak-topeak of 6 volts so that the effective voltage at the grid of control tube VT4 from the sweep oscillator is +-3 volts. This voltage swing changes the frequency of the oscillator tube VT3 by an amount suiiicient to sweep the receiver through a range of frequencies approximately 1% of the carrier frequency. When the director is rendered operative, in a manner to be described, it generates a voltage of i2() volts even on the weakest signal for which the system is designed. This voltage is sufficient to overpower the sweep voltage and to tune the receiver to the incoming carrier within approximately 1 kilocycle, at a carrier frequency of 150 megacycles.

The means for rendering the director operative upon the reception of a carrier modulated by the proper tone frequency comprises the switching tube VTIII which is energized by the tone filter I1. This filter comprises an amplifying tube VT8 whose grid is connected to the audio amplier through a small condenser C1 and the coupled resonant circuits TC3 and TC4 which may be permeability tuned to the desired tone frequency, as indicated at32. Where the identifying tone frequency is within the preferred range of from 4000 to 20,000 cycles, each of these circuits is tuned to that frequency.

'I'he selected tone frequency voltage is amplified by tube VT! in a resistance coupled stage and rectified by the circuit including the diode I8, I3 and resistor RI I, one end of which is grounded. The grid of the audio output tube VTS is normally biased to -10 volts by means of the self-biasing resistor 33 which is shunted by a condenser 34. This arrangement provides a normal negative voltage of 10 volts on the plate IB of the diode with reference to its cathode I9. A signal must, therefore, develop a potential on the positive peaks in excess of the 10 volts negative bias in order to permit the diode to rectify. A weak desired signal which may, for example, be modulated at a tone frequency of 7000 cycles for purposes of station identification, be scanned by the action of the oscillator I2, control device I3 and sweep oscillator I4, and in about 3 milliseconds builds up in the resonant circuits TC3 and 'IC4 to approximately full amplitude. This voltage is amplified by tube VTB and impresses substantially 20 volts positive peak voltage on the diode plate I8, thus overcoming its 10 volts .negative bias and producing a rectified output voltage across RII of the order of 10 volts. This voltage is filtered by the filter circuit comprising resistor RI2 and C8 and will be hereafter referred to as the switch operating voltage.

It is assured that no director voltage is developed when the receiver is passing through the extremely strong undesired carrier by maintaining the screen 25 at a potential of from -20 to 30 volts, which is sufiiicent to absolutely block the tube VTI so that no director voltage is developed in the circuits TCI, TC2. The screen 25 is connected to the plate of the tube VTI 0 which, being unbiased, has a very low resistance and draws such a large current through RI 6 that the screen voltage is about 20 volts above ground. Since the cathode of this tube remains at substantially 50 volts above ground, the screen voltage is 30 volts negative with respect to the cathode.

When a desired signal is encountered and current of the tone frequency builds up the switch operating voltage as before described, this voltage is negative due to the passage of current through the diode I8, I9 and resistor RII. This negative voltage being applied to the grid of tube VTIII is sufiicient to block this tube with the result that the flow of current through resistor RIG is considerably decreased and the voltage of screen 25 immediately rises to about volts above ground and VTI is then operative. The carrier wave of a desired station now being received, resonant voltage are built up in the circuits TCI and TC2. the differential voltage developed being applied to the grid of the control tube VT4. This voltage is sufiicient to override the sweep voltage developed by the sweep oscillator I4 and lock the receiver on the carrier. The noise voltages appearing in the director line 22 are preferably ltered out by the filter circuit which comprises the internal resistance of the director device which is about 2 megohms, and the condenser C4 which has a capacity of 0.5 microfarad.

It is to be noted that both the tone and the carrier must be present to maintain this condiaaeasas Y tion. If either one disappears for more than a few milliseconds, the director driver tube VTI becomes blocked so that no director voltage is developed and the receiver instantly resumes the scanning condition under the control of the sweep oscillator Il.

Should a very strong undesired carrier be encountered having, for example, 6000 cycles modulation on it, the requisite switch operating voltage will not be developed by the resonant circuits TCB, TCL

The voltage developed by the circuits T03, TCI as determined from their combined selectivity curves at 6000 cycles is only about one-tenth of that developed at their resonant frequency of '7000 cycles. But this voltage difference initself would be insuillcient to prevent the development of an effective switch operating voltage which might lock the receiver on an undesired carrier which may be 100,000 times stronger. One of the means which has been found effective to prevent the development of the switch operatingvoltage for an undesired carrier comprises the tube VT'F, the constants of'this tube and its associated circuits being designed so that the maximum power developed in its plate circuit is substantially 100 milliwatts. This amount of energy is insuilicient to develop more than about 7 peak volts across the diode I8, I 9 when a very strong signal modulated at 6000 cycles is encountered since to get any substantial response from a resonant circuit oil its resonant frequency, a considerable amount of power is required to drive it. 'Ihe 'l volts des veloped across the diode is 3 volts less than the minimum voltage required to develop any direct current voltage across RI I so that no output voltage whatsoever is developed in the diode circuit and the tone switch is not operated to render the director effective to lock the receiver on the 6000 cycle modulated carrier.,

When a very strong undesired carrier is swept over, a large transient voltage is developed'as the slope of the resonance curve of the receiver slides past the carrier frequency. This strong transient does, by shock excitation, cause the oscillation of circuits TG3, T04 at their own resonant frequency, but due to the extremely limited driving power provided in the output circuit of tube VT?, only about 5 volts is generated across the circuit T04 after amplification by VT8 which is, ci course, insuiiicient to overcome the biasing volt-1 age of diode I8, I9 so that no switch operating voltage is developed.

The sweep oscillator I i and its associated filter circuit comprising R1 and Cl illustrated develops a substantially sine shaped voltage wave to eiect the scanning action, but it will be understood that a source which generates a voltage wave. of other desired shapes such as triangular, sawtooth, etc., may be used instead, as explained in more detail in the aforementioned application,

A It has been noted after'considerable experience with receivers of this type that as the frequency is raised we' can attain such a frequency that the ordinary noises which prove so troublesome in ordinary reception, such as static, diathermy marecognized by` anyone skilled in the art.

vWhere space and weight areno consideration in designing a receiver of 'this type, we should probably always prefer a definite sweep voltage such as that produced by the 0.5 cycle oscillator shown in Figure 2, but experience has shown that if the switch SW2 be closed, the fluctuating noise voltages produced in the output of the audio stage VTI by the amplification of the thermal agitation and shot effect urrent of the first stage is sunlcient to sweep e control tube through its entire range quite rapidly and at random, butin agiven period we find we cover the entire useful range of sweep in less than a second, although it is recognized that several pulses of the same polarity may follow each other rather than the smooth pulses of always alternate polarity which we obtain from the more disciplined action of the 0.5 cycle oscillator, which is, of course, omitted under this condition. The above condition obtains only when the receiver is of great sensitivity, i. e., so sensitive as to develop on peaks a voltage suincient to fully control the control tube.

The tuning ranges of receivers of the type described are of two distinct classifications: first, for comparatively large tuning ranges that are to be covered, where operation may be expected on any one frequency throughout these large ranges; and, second, where specific point-to-point service might be set up on an assigned frequency. In this latter case, no tuning range at all in the ordinarily accepted sense isy required, but merely excursion around the assigned frequency to establish contact between transmitter and receiver at will. In this latter case probably much closer tolerances can also be expected with respect to the accuracy) of the transmitter frequency, as it would probably be crystal controlled, and 'only the receiver errors would remain. We thus have the two conditions, one a large tuning range and a `small sweep range; the other, no tuning range at all, but a sweep range probably considerably less than 1%. f

In a receiver designed for operation over a tuning range of from 100 to 200 megacycles the following circuit constants are found suitable,

Ri =R2=0.5 megohms R3=R=0-2 megohms Rii=R6=2.0 megohms Rl=10 megohms R8=400-l500 ohms RI 0:25.000 ohms R9=0.5 megohms RI1=125,000 ohms 'The general B+ supply is 250 volts.

Vacuum tubes of the following types are found suitable for use with the circuits described:

VTi=type 6.1i?7 pentode VT2=type 6H6 double diode V'I3=type 955 acorn triode VT4=type 954 acorn pentode VT5, VTO=type 6N? double triode VTl, VT8=type 6N? double triode ln vusing the receiver, the operator tunes the input circuit of the radio frequency amplifler 3 f so that it will pass the carrier frequency or frechines, sparking commutator motors and ignition quenciesof the transmitter or transmitters with which it is desired to establish contact. In the illustrated embodiment this tuning is eiected by manual adjustment of the permeability tuning means 4 which is unicontrolled in any known manner with the tuning means 29 of the oscillator. Since the frequency of oscillator VTS is bedefinite and dependable character and is easily ing cyclically varied through the action of the control tube VT4 whose reactance is in turn being cyclically varied by the sweep oscillator VTS, VTi, as soon as a carrier appears which forms a beat frequency with the oscillator frequency which lies within the frequency band passed by the intermediate frequency amplifier 8, an energizing voltage is applied through lead 23 to the control grid of the director driver tube VTI. If the received carrier is not modulated by the proper tone frequency, the tube VTI is blocked in the manner above explained and no director voltage is built up in the circuits TCI, TC2, so that the receiver is not locked on the carrier, but the scanning action is continued under the control of the sweep oscillator Il. Upon receiving a desired carrier which is modulated with the tone frequency to which the tone filter circuits TO3, TCI are resonant, a tone switch operating voltage is developed across the diode resistor RII which is sufficient to substantially block the tube VTIU. This action causes the voltage of screen 25 to rise very substantially, so that the plate current of VTI energizes the director VT2, and a minimum director voltage of 20 volts is developed and through line 22 is applied to the control grid of the control tube VT4. This high voltagel overpowers the control voltage of :t3 volts developed by the sweep oscillator and serves to maintain the reactance of the control tube at a constant value, thereby maintaining the oscillator frequency at a constant value which heterodynes with the carrier frequency to produce the proper beat frequency which lies within the band of frequencies passed by the intermediate frequency amplifier 6. 'Ihe receiver remains locked on the carrier as long as the tone modulated carrier is received. If the carrier disappears, no voltage is supplied through line 23 to energize the director with the result that no director voltage is developed and the receiver resumes the scanning condition under the control of the sweep oscillator I4. If the carrier continues to be received but the tone modulation disappears, no currents are available of the tone modulation frequency in the output circuit of tube VTI, so`

that no switch operating voltage is developed by the tone filter. This causes the tube VTIU to draw a heavy plate current, and the voltage of screen 25 to drop low enough to block the director driver tube VTI. Since no director voltage is now developed or applied to the control grid of the control tube, the sweep oscillator resumes control of the control tube and the scanning opf eration is resumed.

The ability of this receiverto respond only to a carrier designated by a given tone allows the frequency of the transmitted carrier to be shifted at will over rather large ranges of frequencies, and, since the receiver is locked on the carrier, it would faithfully follow such changes 1f they are continuous. If the carrier frequency were varied in a smooth and continuous manner, some difficulty would ensue if it happened to pass over an undesired carrier, so it is preferred to cause the carrier frequency to vary in a discontinuous manner. The receiver, in reestablishing contact after such a discontinuity in frequency, has the ability of passing over an undesired carrier, which it might encounter in passing to the new carrier frequency, without any slowing down in its scanning action. It is accordingly one of the purposes of this invention to provide a scanning receiver in which the scanning and identifying action is so rapid that a continuous communication may be kept up on the several separate carrier frequencies alternately without any noticeable lapse. One field of use for a receiver of this type is in the establishing of communication where secrecy is desired.

Where a receiver of this type is to be used in point-to-point communication where there is no possibility that the carrier wave will seriously weaken once contact has been established, it is perfectly feasible to. operate the receiver in the tone lock, carrier hold condition. In this condition the tone is impressed on the carrier only for a second or so to be sure the receiver has had time to lock on the designated carrier, and the tone is then removed leaving the receiver locked on the carrier. Means for doing that include the connection 35 and the switch SWI. Upon closure of this switch the receiver is placed in the tone lock, carrier hold condition, since any negative voltage developed across RI will be applied to the grid of VTIII, the switch tube.

Assuming a receiver to have scanned its usual portion of the spectrum, and encountered a carrier having a proper tone to which TG3 and TCI are tuned, the necessary switch operating voltage will appear across RII and be applied to VTIII. While the receiver is scanning, of course, VTI is blocked and no voltage can appear across RI. However, once VTIII switch tube has been blocked, VTI in turn is unblocked and a negative voltage builds up across RI due to the carrier wave. This negative voltage is immediately applied to the grid of VTI!) through the connection 3B and is of suilicient amplitude to maintain VTIII in a fully blocked condition. If the tone is now removed from the carrier, the voltage from RI will hold the system in operative condition until such time as the carrier wave disappears. It is obvious that once the carrier disappears, the receiver resumes the scanning condition and will again lock only when a carrier identified by the proper tone is encountered. The term scanning receiver as used in the claims designates a radio receiver which automatically explores a plurality of communication channels in succession lying between two definite frequencies in the spectrum. 'I'he term inertialess" as used in the claims designates a control means having no moving mechanical parts and hence free of mechanical inertia.

While the receiver shown in Figs. 1 and 2 and above described is a single superheterodyne, the apparatus in which the invention was developed was a double detection type superheterodyne with a rst intermediate frequency of 17.5 megacycles and a nal intermediate frequency of 0.46 megacycle.

I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be confined to the embodiments shown, but what I-desire to cover by Letters Patent is set forth in the appended claims.

I claim:

l. A radio receiver adapted to receive a tone modulated carrier wave and comprising a circuit tunable over a range of frequencies but normally untuned to the carrier frequency, automatic means for continuously tuning said circuit over the frequency range, and automatic means responsive to the tone modulations of a single continuous impulse of the tone modulated carrier for rapidly arresting the action of said tuning means and for maintaining said circuit tuned to a desired frequency within said range of frequencies.

2. A radio receiver arranged to rapidly and automatically scan a plurality of adjacent communication channels, the carrier wave of one channel being modulated at diii'erent times at different predetermined tone frequencies, said receiver comprising means for detecting the modulation frequency, a pair of tone filters connected tosaid detecting means and each responsive to one of said tone frequencies, and means connected to both of said tone filters and operative upon the reception of a single continuous pulse of a carrier modulated by either of the tone frequencies to rapidly arrest the scanning action of the receiver and lock it on said tone modulated carrier.

3. A radio receiver for receiving a tone modulated carrier wave comprising, in combination, a tunable circuit, means for automatically and continuously tuning said circuit over a range yoi frequencies as wide as a plurality of communication channels, a diode having an input circuit, a tuning control'device connected to the diode, and means connected to the diode for causing said tuning control device to rapidly arrest the continuous tuning of said circuit and to maintain the circuit resonant at a desired frequency in response to the impression of voltages of the tone modulation frequency of the carrier on the input circuit of the diode.

4. In combination with a radio receiver of the superheterodyne type for receiving a tone modulated carrier wave and having an oscillator and a second detector, a diode having an input circuit coupled to the output of said second detector, a tuning control device connected to said diode and to theinput of said detector and including a reactance control tube connected to said oscillator, and means connected to the diode for causing said tuning control device to maintain the frequency of the oscillations generated by the oscillator at a desired value in response to the impression of voltage ofthe tone modulation frequency on the input circuit of the diode by the second detector.

5. A scanning receiver having means for automatically tuning it over a plurality of communication channels and means for automatically ar- Y resting the scanning action thereof and for rendering said receiver continuously responsive to a tone and signal modulated carrier wave, said means comprising a driver tube having an output circuit of limited power output and comprising a device resonant to the tone modulation frequency of the carrier, and a detecting device coupled to said output circuit and having the property of being substantially responsive only to voltages generated in the responsive device which are greater than a predetermined value.

6. A radio receiver for a transmitted carrier Wave having tone and signal modulations, said receiver being tunable over a range of frequencies including a plurality of carrier frequency channels, means free of mechanical inertia for cyclically and automatically tuning said receiver over the frequency range, control means responsive to the reception of the carrier frequency, and means responsive to the tone modulations of the carrier for rendering said control means effective to maintain the receiver tuned to a desired frequency.

7. In combination with a radio receiver of the superheterodyne type for receiving tone modulated carrier waves of different frequencies within a'band of frequencies, said receiver comprising a first detector and an oscillator, means for automatically and cyclically varying the frequency of the currents generated by said oscillator throughout a range substantially .equal to said band, means for detecting the modulations of the carrier, and means coupled to said detecting means and responsive to the tone modulations of a Adesired carrier for causing the frequency of the currents generated by the oscillator to remain at a desired value.

8. In combination with a radio receiver of the superheterodyne type for receiving tone modulated carrier Waves and comprising an intermediate frequency amplifier, a detecting device connected to the intermediate frequency amplifier and adapted to detect the carrier modulations and an electron discharge tube oscillator having a grid circuit and a plate circuit tunable over a range of frequencies, a rea'ctance generating control tube having a plate circuit and a grid circuit, means connecting the plate circuits of said tubes in parallel, means for generating a fluctuating voltage coupled to the grid circuit of said control tube, a frequency responsive director device continuously connected to a grid of said control tube and adapted to control the transconductance of said tube, a switching device operative to selectively connect said director device to the intermediate frequency amplifier, and means connected to said detecting device and including a filter circuit responsive to the tone modulations of a. desired carrier wave for operating said switching device to thereby cause the director device to be energized by the amplified intermediate frequency currents.

9. A radio receiver adapted to receive tone modulated signal currents of different carrier frequency channels within a band of frequencies higher than 20 megacycles and comprising,I in combination, means free of mechanical inertia operative to automatically and cyclically tune said receiver over the frequency band, and means responsive to the reception of a tone modulated carrier having a frequency within said band for rendering said means ineffective to continue the tuning of the receiver over said band and to automatically permit the cyclic tuning of the receiver by the tuning means again, in response to the disappearance of the tone modulations of the carrier.

10.' A radio receiving system comprising, in combination, a radio receiver adapted to receive tone modulated signal currents, said receiver having a high order of sensitivity and being tunable over a range of frequencies :higher 'than 20 megacycles including a plurality of communication channels, an amplifier coupled to the output of said receiver and having a fluctuating noise current component when the receiver is untuned to a carrier frequency within said range, means energized by the noise component of the amplifier for causing the random tuning oi the receiver over said range of frequencies and means responsive to the tone modulation 'of a carrier for stopping the random tuning of the receiver and for locking it on such tone modulated carrier.

11. A radio receiving system for tone and signal modulated carrier waves, said system coniprising, in combination, an input circuit, a communication channel, a detector device connected to said input circuit for detecting the tone modulations of the carrier, automatic means for cyclically varying the frequency of the signaling energy impressed on said communication channel by the input circuit at a substantially steady rate in one direction and at a more abrupt rate in the opposite direction, and means connected to the output of said detecting device and responsive to the tone modulations of a carrier for overcoming the effect of said automatic frequency varying means and for maintaining the frequency of the signaling energy impressed on the communication channel at a desired value.

12. A radio receiver for a carrier having tone and signal modulations, said receiver comprising, in combination, an input circuit including an amplifier, a communication channel, automatic means for automatically varying at a rapid rate the frequency of the signaling energy impressed on said communication channel by the input circuit, an automatic volume control device arranged to vary the amplification of said amplifier inversely as the strength of a received carrier and having a time constant of such value that the amplification of the amplifier is not substantially reduced upon the reception of another carrier passed over at the rapid rate and unmodulated by the tone modulations, and means responsive to the tone modulations for maintaining the frequency of the signaling energy impressed on the communication channel at a desired value.

13. A receiver adapted for the continuous reception of a signal transmitted in sequence on at least two different carrier frequencies each of which is modulated by a predetermined tone, said receiver comprising a tunable input portion, means for tuning said input portion to one of the carrier frequencies, means for automatically tuning said input portion to the other carrier frequency and for maintaining said input portion tuned to said other frequency, and means for rendering said last named means ineffective to maintain said tunable input portion tuned to a carrier frequency unmodulated by the predetermined tone.

14. A scanning receiver for the reception of tone modulated carrier waves of different frequencies, the receiver being normally untuned to a desired carrier comprising, in combination, means responsive to a predetermined tone for locking the receiver on a carrier modulated by said predetermined tone, and means responsive to currents of the carrier frequency for maintaining the receiver locked on the carrier upon `the disappearance of the tone modulations.

SIDNEY Y. WHITE. 

